This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power for vehicle propulsion.
Fuel cells have been used as a power source in many applications and have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively December 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell depends on the ability to effectively disperse reactant gases at catalytic sites of the electrode where reaction occurs. In addition, effective removal of reaction products is required so as to not inhibit flow of fresh reactants to the catalytic sites. Therefore, it is desirable to improve the mobility of reactant and product species to and from the MEA where reaction occurs.
The present invention contemplates a diffusion structure which enhances mass transport to and from an electrode in a membrane electrode assembly (MEA) of a fuel cell. The diffusion structure cooperates and interacts with an electrode at a major surface of the electrode opposite the membrane electrolyte of the cell. The diffusion structure is a composite diffusion medium which facilitates the supply of reactant gas to the electrode. The diffusion structure also facilitates movement of water. The diffusion structure includes a characteristic bulk layer having two or more portions, each with properties defined below, including hydrophobicity and surface energy. The bulk layer is useable alone to function as a diffusion structure. However, it is preferably combined with an absorption layer and a desorption layer on respective sides of the bulk layer to form a preferred diffusion structure.
The diffusion structure preferably comprises an absorption layer which has a first electrically conductive material. The absorption layer has a surface facing or engaging the major surface of the electrode structure; and the absorption layer accepts water from the electrode structure. Water is a product of the reaction in the cell between hydrogen and air at the cathode.
The diffusion structure also comprises the bulk layer which has a second electrically conductive material. The bulk layer has a surface facing or engaging a major surface of the absorption layer opposite the electrode structure. The bulk layer has at least two portions, the first portion is less hydrophobic than the second portion. The first portion is nearest the absorption layer.
The diffusion structure preferably further comprises a desorption layer which has a third electrically conducted material. The desorption layer has a surface facing or engaging the second portion of the bulk layer, and an opposite surface facing away from the electrode structure. Water is released at this opposite surface of the desorption layer.
Preferably, the bulk layer comprises at least one intermediate portion between the first and second portions, where the hydrophobicity of each of the intermediate portions is greater than the first portion and less than the second portion. Preferably, the hydrophobic character of each intermediate portion is selected so that hydrophobicity increases in a direction away from the membrane electrode assembly. Preferably, a plurality of intermediate layers is arranged between the first and second portions, with decreasing surface energy and increasing hydrophobicity in the direction from the first portion to the second portion. Preferably, the diffusion structure is further characterized by increasing hydrophobicity and by decreasing surface energy in a direction from the electrode toward the opposite surface of the desorption layer.
In another aspect of the invention, specific materials are selected for the absorption layer, the bulk layer, and the desorption layer to provide the properties of surface energy, hydrophobicity, and corresponding hydrophilicity to optimize movement of reactant gases in a direction toward the membrane electrode assembly and to move product gases and water in a direction away from the membrane electrode assembly. Accordingly, the absorption layer preferably comprises the first electrically conductive material dispersed in a fluorinated polymeric binder (PVDF). The bulk layer first portion preferably consists essentially of the second electrically conductive material. The second portion of the bulk layer comprises the second electrically conductive material intermingled with polytetrafluoroethylene (PTFE). The amount by weight of the PTFE is less than the amount of the electrically conductive material in the second portion. The desorption layer preferably comprises the third electrically conductive material intermingled with PTFE, and the amount of PTFE relative to the third electrically conductive material is greater than the amount of PTFE relative to the second electrically conductive material in the second portion of the bulk layer. In one aspect, the three electrically conductive materials differ from one another.
In another aspect, the characteristics of the layers are further understood by reference to designated numbered surfaces of the layers. The electrode structure has a first surface facing or engaging the electrolyte for forming a part of the MEA. The second surface of the electrode structure faces or engages the absorption layer""s third surface. The absorption layer""s fourth surface faces or engages the fifth surface of the bulk layer and the bulk layer""s sixth surface faces or engages the seventh surface of the desorption layer. The desorption layer""s eighth surface is furthest away from the MEA. Here, the bulk layer comprises at least two portions, the first portion of the bulk layer is adjacent its fifth surface and the second portion of the bulk is adjacent its sixth surface. The first portion has a material with a surface energy greater than the surface energy of the material of the second portion. Here, decreasing surface energy is provided between the fifth and sixth surfaces of the bulk layer. As described above, the bulk layer preferably comprises at least one intermediate portion between the first and second portions. The surface energy of the materials of each intermediate portion is between that of the surface energy of the material of the first portion and the material of the second portion.
Preferably, the surface energy of the material of the fourth surface of the absorption layer and the fifth surface of the bulk layer are approximately the same. Preferably, the surface energy of the materials of the sixth surface of the bulk layer and the seventh surface of the desorption layer are approximately the same and distinctly different from the surface energies of the materials of the fourth and fifth surfaces. Preferably, the absorption layer, bulk layer, and desorption layer are formed of materials which provide decreasing surface energy between the second surface of the electrode structure and the eighth surface of the desorption layer which is the surface of the diffusion structure furthest away from the MEA.
In another aspect, the invention provides a diffusion structure having the characteristic of the bulk layer which has at least two portions which are characterized by decreasing surface energy and increasing hydrophobicity in a direction from the electrode surface outward. The bulk layer, having features as described above, is used alone or in combination with any of the absorption and desorption layers described above. Therefore, if desired, an absorption layer is interposed between the bulk layer and the electrode layer. If desired, a desorption absorption layer is used in combination with the bulk layer. Although it is possible to use the bulk layer with its specific characteristics as the only layer for transport of reactive gases, product gases and particularly water, it is preferred to use the bulk layer in combination with an absorption layer which is formed of an electrically conductive material that is different from the electrically conductive material provided in the bulk layer. The combination is further enhanced by the use of the desorption layer which comprises another electrically conductive material which is different from the electrically conductive material provided in the bulk layer and the absorption layer.
The present diffusion structure arrangement when used in combination with a membrane electrode assembly and particularly the cathode structure of such assembly, effectively disperses reactant gases at the catalytic site of the electrode where reaction occurs. In addition, the diffusion structure effectively removes reaction products, particularly water, so as not to inhibit flow of fresh reactants to the catalytic site. Therefore, the diffusion structure of the invention provides the desirable features of improved mobility of reactant and product species to and from the MEA to facilitate and enhance its performance.